In May 2007, 42 Entertainment began a viral marketing campaign utilizing the film's "Why So Serious?" tagline with the launch of a website featuring the fictional political campaign of Harvey Dent, with the caption, "I Believe in Harvey Dent". The site aimed to interest fans by having them try to earn what they wanted to see and, on behalf of Warner Bros., 42 Entertainment also established a "vandalized" version of I Believe in Harvey Dent, called "I believe in Harvey Dent too", where e-mails sent by fans slowly removed pixels, revealing the first official image of the Joker; it was ultimately replaced with many "Haha"s and a hidden message that said "see you in December".
During the 2007 San Diego Comic-Con International, 42 Entertainment launched WhySoSerious.com, sending fans on a scavenger hunt to unlock a teaser trailer and a new photo of the Joker. On October 31, 2007, the film's website morphed into another scavenger hunt with hidden messages, instructing fans to uncover clues at certain locations in major cities throughout the United States, and to take photographs of their discoveries. The clues combined to reveal a new photograph of the Joker and an audio clip of him from the film saying "And tonight, you're gonna break your one rule." Completing the scavenger hunt also led to another website called Rory's Death Kiss (referencing the false working title of Rory's First Kiss), where fans could submit photographs of themselves costumed as the Joker. Those who sent photos were mailed a copy of a fictional newspaper called The Gotham Times, whose electronic version led to the discovery of numerous other websites.
The Dark Knight's opening sequence, (showing a bank raid by the Joker) and closing montage of other scenes from the film, was screened with selected IMAX screenings of I Am Legend, which was released on December 14, 2007. A theatrical teaser was also released with non-IMAX showings of I Am Legend, and also on the official website. The sequence was released on the Blu-ray Disc edition of Batman Begins on July 8, 2008. Also on July 8, 2008, the studio released Batman: Gotham Knight, a direct-to-DVD animated film, set between Batman Begins and The Dark Knight and featuring six original stories, directed by Bruce Timm, co-creator and producer of Batman: The Animated Series, and starring veteran Batman voice actor Kevin Conroy. Each of these segments, written by Josh Olson, David S. Goyer, Brian Azzarello, Greg Rucka, Jordan Goldberg, and Alan Burnett, presents its own distinctive artistic style, paralleling numerous artists collaborating in the same DC Universe.
After the death of Heath Ledger on January 22, 2008, Warner Bros. adjusted its promotional focus on the Joker, revising some of its websites dedicated to promoting the film and posting a memorial tribute to Ledger on the film's official website and overlaying a black memorial ribbon on the photo collage in WhySoSerious.com. On February 29, 2008, I Believe in Harvey Dent was updated to enable fans to send their e-mail addresses and phone numbers. In March 2008, Harvey Dent's fictional campaign informed fans that actual campaign buses nicknamed "Dentmobiles" would tour various cities to promote Dent's candidacy for district attorney.
On May 15, 2008, Six Flags Great America and Six Flags Great Adventure theme parks opened The Dark Knight Coaster roller coaster, which cost US$7.5 million to develop and which simulates being stalked by the Joker. Mattel produced toys and games for The Dark Knight, action figures, role play costumes, board games, puzzles, and a special-edition UNO card game, which began commercial distribution in June 2008.
Warner Bros. devoted six months to an anti-infringement strategy that involved tracking the people who had a pre-release copy of the film at any one time. Shipping and delivery schedules were also staggered and spot checks were carried out both domestically and overseas to ensure illegal copying of the film was not taking place in cinemas. An unlicensed copy was released on the web about 38 hours after the film's release. BitTorrent search engine The Pirate Bay taunted the movie industry over its ability to provide the movie free, replacing its logo with a taunting message.
Pandemic Studios was developing a tie-in video game adaptation, Batman: The Dark Knight. However, its development faced a series of disruptions and was canceled before completion.